1. Field of the Invention
This invention relates to video to film conversion.
2. Description of the Prior Art
For some years now it has been possible to transfer video to film using an electron beam recording (EBR) system. When converting HDVS 60 Hz (60 fields/s) 2:1 interlace format video to 24 Hz (24 frames/s) film using this system, a video signal corresponding to 24 frame/s is produced from the 60 Hz video signal, and the 24 Hz video signal is used to drive an electron beam recorder for recording the 24 Hz frames on film. The frame rate conversion is achieved in this system by a "field-combined drop-field" process.
In this process, two output film frames are produced from every five input video fields. The first and second input fields are "field combined" to produce a first video frame, the third and fourth input video fields are "field combined" to produce a second video frame, the fifth input video field is dropped or ignored, and the first and second video frames are recorded on film by the electron beam recorder. By field combining, it is meant that the odd lines of the video frame are made up of the lines of the odd input video field, and the even lines of the video frame are made of the lines of the even input video field.
The drop field process is a very crude form of standards conversion which introduces a 12 Hz judder component due to the loss of every fifth field of the original 60 Hz video. There is also a reduction in dynamic resolution (or an introduction of motion smearing) since temporally offset fields are combined to produce each frame of the film so that the outline of moving objects can appear more blurred than the blur caused by camera integration effects alone. This additional motion blur will be referred to as "interlace smear". Thus, while the field-combined drop-field system offers picture quality which is adequate for the majority of the time, motion rendition is poor due to the 12 Hz judder in addition to the 24 Hz strobing introduced by the film projector, and there is reduced dynamic resolution.
In order to overcome the above problems, an elaborate conversion system has been developed involving "motion compensated temporal interpolation". To convert 60 Hz 2:1 format video to 24 Hz film, the process involves: (i) producing progressive scan frames at 60 frames/s from one or three of the input 60 fields/s fields depending upon motion in the input fields; (ii) detecting motion in areas of the image between pairs of temporally adjacent progressive scan frames; (iii) producing output frames at 24 frames/s, four for every ten progressive scan frames, with each pixel of each output frame derived from pixels in a respective pair of the progressive scan frames spatially displaced from the output pixel in dependence upon the detected motion and the temporal misalignment between the output frame and the pair of frames from which is it formed; and (iv) recording the output frames on film. Thus, the motion in the image is analysed and interpolation of each pixel is performed along the direction of motion.
Such a system for converting 60 Hz 2:1 interlace format video to 24 Hz film is described in detail in patent application GB 2231228A. The system gives output pictures of extremely high quality in which dynamic resolution is preserved and motion is very smooth. However, the process can be labour intensive, and the equipment is complex, bulky and expensive.
In a different technical field of film-to-film conversion, it is known from patent specification U.S. Pat. No. 4,889,423 to convert 60 frame/s film to 24 frame/s film using an optical process. In a series of five input frames of the 60 frame/s film and two corresponding output frames of the 24 frame/s film, the image of the first output frame is produced so that one portion of its exposure includes the image of the first input frame and the other portion of its exposure includes the image of the second input frame. The third input frame is not used; and the image of the second input frame is produced so that one portion of its exposure includes the image of the fourth input frame and the other portion of its exposure includes the image of the fifth input frame.